Today, there are many radio and cellular access technologies and standards such as GSM/GPRS (Global system for mobile communications/General Packet Radio Service), WCDMA/HSPA (Wideband Code Division Multiple Access/High Speed Packet Access), CDMA (Code Division Multiple Access)-based technologies, WiFi (Wireless Fidelity), WiMAX (Worldwide Interoperability for Microwave Access) and recently LTE (Long Term Evolution), to name a few. The technologies and standards have been developed during the last few decades, and it can be expected that the development will continue. Specifications are developed in organizations like 3GPP, 3GPP2 and IEEE. 3GPP is responsible for the development and maintenance of GSM/GPRS, WCDMA/HSPA and LTE standards.
Various frequency bands are typically allocated and/or sold by government organizations, such that an operator may “own” certain bands for a particular use (i.e. the right to use the band in a certain way). Regulations may specify that the owner, i.e. the operator, should deploy a particular technology in a particular frequency band. In some cases, the operator may be able to choose what technology and standard to deploy in their spectrum provided that the choices fulfill certain criteria set up by e.g. the ITU (International Telecommunications Union).
As a consequence of the fact that spectrum is a scarce resource, an operator may have the rights to deploy a new cellular access, such as LTE, in a limited spectrum of, say 20 MHz.
However, the fact that the operator may have an existing customer base with existing terminals will prevent the operator from deploying only one technology in the whole spectrum owned by the operator. This could be the case e.g. for an operator that has a large customer base with WCDMA/HSPA subscriptions using the Universal Terrestrial Radio Access Network (UTRAN), and the operator wants to deploy the most recent evolution, the Long Term Evolution (LTE) of UTRAN, also called evolved UTRAN or E-UTRAN.
In this example, the operator may then have to divide the available bands between HSPA and LTE. At initial deployment of LTE, the operator may thus continue to use e.g. 10 MHz (corresponding to two WCDMA carriers) with HSPA and reserve 10 MHz for initial LTE deployment.
However, such partitioning of the scarce spectrum to different technologies has some undesired effects on performance:                There is a direct correlation between the peak-rate that can be offered and the spectrum width that is used. Thus, limiting the bandwidth of both HSPA and LTE to 10 MHz in the example above will roughly limit the peak-rate offered to customers to a half. Thus, assuming now, for the sake of illustration, that the technologies can offer around 100 Mbps in 20 MHz, it will mean that the peak-rate will now be limited to around 50 Mbps in each of the technologies.        Initially, it may happen that the HSPA carriers are very loaded, while the LTE carriers in the example only have a few users. Thus, there would be an imbalance between allocation and usage resulting in undesired congestion on the HSPA carriers. However, in order to offer a decent bit-rate on the LTE carriers, it is still not possible to allocate e.g. only 5 MHz to LTE customers, since then the LTE evolution would not provide competitive performance in relation to HSPA.        
There have been discussions to find a solution for simultaneous use of multiple radio access technologies. Carrier aggregation (CA), wherein a combination or aggregation of two independent carriers is made, is one way of achieving increased resource utilization and spectrum efficiency. For example, in LTE+HSPA carrier aggregation each carrier may be an LTE carrier or a HSPA carrier. Higher peak rates and load balancing can be offered in heterogeneous deployments including at least two radio-access technologies. Both LTE carrier aggregation as well as HSPA carrier aggregation, i.e. carrier aggregation within the same Radio Access Technology (RAT), is defined in the 3GPP specifications. Dual-carrier HSPA was specified in the Release 8 standard, and LTE carrier aggregation was specified in the Release 10 standard of the 3GPP specification.
A terminal or user equipment (UE) needs to monitor the radio link or links to/from the network (NW) node, such as a base station or NodeB/eNodeB in order to detect problems with the connection or the connectivity. Depending on the system/RAT used different actions are needed in response to e.g. Radio Link Failure (RLF).
Some details of radio link monitoring in WCDMA/HSPA are described in 3GPP TS 25.331, e.g. Clause 8.5.6, and are only briefly described here for reference. In WCDMA/HSPA, using closed loop power control, the uplink (UL) power (from UE to network node) is controlled by the network node and information to increase/decrease power is sent to the UE via transmit power commands (TPC). The TPC do not have any error correction coding, hence the UE interprets the TPC and increases or decreases its transmitted power based on detected TPC. This is made 1500 times per second, i.e. power variations of 1500 dB/second are possible (although limited to a certain range of −50 to 24 dBm). The UE transmission power will have a large impact on the UL performance in neighbor cells within the proximity of the UE, causing additional interference in the neighbor cells. This is particularly true in case of an unreliable downlink (DL), in which case unreliable decoding of the TPC commands could result in that the UE is transmitting with a power that is unsuitable for the current conditions. Therefore, the UE needs to monitor reliability of the TPC commands and in case of unreliable TPC commands on the DL channels, the UE turns off its transmitter in order to not generate interference in the uplink channels. This also holds for the dual-cell HSPA case, when multiple UL carriers or channels are configured. The specifications describe various timers during which physical layer problems are detected and during which a recovery may be observed without resorting to further actions in the UE. If the layer 1 error conditions prevail, the UE will need to release the existing radio link(s).
In LTE however, the network is in control of how the UL resources on the shared UL channel or channels are allocated to each UE, and the TPC transmitted to the UE for UL power control are decoded for error detection. Hence an unreliable DL in LTE will not introduce significant UL interference, but in case the decoding of the commands sent from the network on the DL is unreliable, and if the DL is a primary downlink channel, the UE should anyway disable its UL transmission. It is also important that the UE can select another cell, in case it cannot reliably decode any transmissions from the cell to which it is currently connected. If the UE would not select another cell, it could happen that the UE would remain in a situation where transmission and signaling is impossible. That is, the UE would remain connected to a cell which it cannot reach and/or the network cannot reach the UE via that cell. 3GPP TS 36.331 Clause 5.3.11 describes in some further detail how a failing link is detected, and what actions the UE should undertake if the conditions prevail.
In case of multi-carrier LTE, however, with multiple UL carriers configured and activated, the UE does not need to deactivate secondary (component/addition) UL carriers (of Secondary Cells, SCells). This is because the network node (such as a base station or eNodeB (eNB)) is in full control of the UE as long as the primary carrier (of Primary Cell, PCell) is ok. That is, the UE can be reached and the UE can reach the network as long as at least one DL/UL carrier pair is useful for communication with the network. In 3GPP Rel-10, it was therefore decided that the UE can refrain from any autonomous actions related to recovery from a radio link failures, as long as the PCell and the corresponding UL carrier used for e.g. random access is found to be acceptable by the UE.
For Time Division Duplex (TDD), the UE receives timing information from DL. This timing information is used for the transmission timing of the UL transmission, and it is noted that it is essential for the UE to be able to keep UL transmission timing within guard periods. The loss of the reference downlink should therefore cause the UE to not transmit on the UL on the carrier, as it could result in severe interference in neighboring subframes. Otherwise, this means that a loss of timing could, in TDD, result in that a UE would partly transmit in a subframe intended for downlink reception, resulting in severe interference to other UEs nearby that are prepared to receive a downlink transmission in such a subframe.
The timers and counters related to radio link monitoring and radio link failure (RLF) in EUTRAN (LTE) are here listed for reference (see TS 36.331, v10.0.0):
Detection of physical layer problems in RRC_CONNECTED                Upon receiving N310 consecutive “out-of-sync” indications for the PCell from lower layers the UE starts timer T310; Upon receiving N311 consecutive “in-sync” indications for the PCell from lower layers while T310 is running, the UE shall stop timer T310;        Upon T310 expiry initiate the connection re-establishment procedure;        
It is noted that physical layer monitoring and related autonomous actions do not apply to SCells.
There are also specific timers for                T301 start: Transmission RRCConnectionReestablishmentRequest, T301 stop: response. Upon T301 expiry: UE enter RRC Idle of at NW        T304 start: Reception of RRCConnectionReconfiguration message including the MobilityControl Info, T304 stop: at handover successfully completed, Upon T304 expiry: UE enter RRC Idle        T311 start: Upon initiating the RRC connection re-establishment procedure stop: selection of suitable cell, Upon T311 expiry: UE enter RRC Idle        
Although prior art method and apparatus exist for radio link monitoring in case of a particular RAT, there is no such method and apparatus describing how and when the terminal/UE should monitor radio links if several RATs are used in the communication between the network node and the terminals. Hence there is a need for such methods describing how radio links should be monitored and corresponding actions when multiple RATs used.